In the first inning of the Mets’ 6-4 loss to the Cardinals, hours after manager Mickey Callaway revealed that Alonso begged him Saturday night to put him in the lineup against St. Louis righthander Dakota Hudson, Alonso crushed a homer to straightaway center against Hudson, his old college foe.

Major League Baseball’s ball-tracking technology said it landed 444 feet from home plate — a conservative estimate — beyond the berm that serves as the batter’s eye in centerfield at Busch Stadium. It came off the bat at 114.5 mph.

And Alonso really wanted this one. “I guess he faced this guy in college,” Callaway said Sunday morning. “He called me last night and said, ‘You better put me in the lineup.’ ”

Callaway was having dinner with his parents when Alonso texted and called. They eventually ran into each other at the team’s hotel.

“I got scared, man. Like, what happened?” Callaway recalled with a laugh. “And then I saw him and then he was like, ‘Hey, I want to play tomorrow. I hate this guy. I played against him in college.’ I don’t know what he was saying. Going nuts . . . He obviously didn’t like him very much. Something like that.”

They were Southeastern Conference rivals — Alonso at Florida and Hudson at Mississippi State — as well as 2016 draft picks, with Alonso going in the second round and Hudson in the first. Alonso was 0-for-4 against Hudson in college and 1-for-3 in a Cape Cod League game.

Afterward, Alonso said he mostly wanted to make sure Callaway wasn’t planning to sit him after he got hit on the hand by a pitch Saturday.

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He clarified that there is no animosity between the two but that Hudson had thrown well against him in the past: “He shoved it every single time.”

Alonso thought going in that he had a pretty good personal scouting report. “Do your homework, kids,” he said.

He leads the Mets — and all major-league rookies — with eight homers and 19 RBIs.

Said Callaway, “He can call me any time he wants.”

Frazier time

Todd Frazier (strained left oblique) tweeted that he was leaving Syracuse on Sunday night, suggesting his rehab assignment is over. He was 0-for-2 with a walk and a strikeout for the Triple-A Mets, splitting time between shortstop (four innings) and third base (five innings). He hasn’t played for the Mets this season because of the oblique injury, which he suffered early in spring training.